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The Author, of the Central Institute for Industrial Research, Oslo, discusses the problems involved in using heavy fuels in ships' Diesel engines and boilers, and gives some information additional to that in his earlier article. Both articles are based on the results of a Norwegian project in which seven heavy fuel oils from various souces were studied; tests, mostly short-term, were carried out on the fuels in combustion test-rigs, boilers, and Diesel engines. The present article is arranged under the main headings: Heavy Fuel Oils and their Properties; Selection of Test Fuels; Discussion of Gas-Chromatographic Analyses versus Boiler Tests; Fuel Oil Treatment (Combustion processes--Mechanical treatment--Additives--Coatings). This Norwegian project has shown that (a) the present "standard" fuel oil analyses as given in the bunker receipt are inadequate for predicting combustion properties; (b) gas-chromatographic analyses can give a better prediction of combustion properties; (c) more information on the composition of heavy fuel oils is necessary; (d) mechanical pre-treatment is one way of improving fuel quality; (e) separation of salt water from water/oil emulsions must be carefully done with the right emulsion-breakers, or serious problems will occur; (f) classification of additives according to their chemical composition is useful for assessing their ability to reduce problems in service; (g) trials have shown that, depending on fuel quality, additives may be successfully used in boiler fuels; (h) wear and corrosion in Diesel engines can be reduced by coating the thermally-stressed parts with a material more resistant than the base metal. A more general conclusion is that the successful solution of fuel problems needs close co-operation between all parties concerned. Order from BSRA as No. 49,955.